I am not a big fan of answering licensing and support issues in a forum. Anyway. Quoting from the Oracle Store:

Pricing metric is per system. The system can have a maximum of two physical processors (sockets), unlimited number of cores and/or virtual guests.
There is no license cost associated with Oracle VM. Pricing reflects $0 for license, and all charges are related to services.

So you don't have to purchase any licenses but you have the option of purchasing support. The licensing metric is per system (so the number of cpus, cores or virtual machines does not matter). The one I quoted above is for the "limited" suport version that has a maximum of two sockets per system, there is also another option for an unlimited number of sockets per system. And if I recall correctly, you do not need to purchase support for the OVM manager machine.

So in your case, you could purchase 3 times the "OVM Premier Limited Support" option https://shop.oracle.com/pls/ostore/f?p=dstore:product:0::NO:RP,6:P6_LPI:4510279062361805728469